Types Of Vision Loss Explained

According to the Centers for Disease Control and Prevention (CDC), around 12 million Americans aged 40+ have a problem with their vision. Vision loss is also one of the 10 most common disabilities affecting American adults (via the CDC). Globally, the World Health Organization (WHO) reports that at least 2.2 billion people have a vision impairment.

Vision loss can take many forms. Loss of central vision can cause blurriness or a blind spot in the center of your field of view (via cavision.org). Peripheral vision loss (also known as tunnel vision) affects only the outer edges of the field of view, but can negatively impact mobility. Some vision loss causes a general blurriness or haziness over the entire field of view, while others may cause extreme light sensitivity or the inability to see in low light (night blindness).

If someone's visual acuity is very poor, they may meet the definition for having low vision or being legally blind. According to the American Federation for the Blind, low vision is a condition in which "visual acuity is 20/70 or poorer in the better-seeing eye and cannot be corrected or improved with regular eyeglasses." Legal blindness, on the other hand, is "a visual acuity of 20/200 or less in the better-seeing eye with best conventional correction (meaning with regular glasses or contact lenses)."

How vision works

As the American Academy of Ophthalmology explains, your inner eyelids and your eyeball are covered with a clear membrane called the conjunctiva, which is kept lubricated by tears. Under the conjunctiva is the sclera, or white of the eye. At the front of the eye is a rounded structure called the cornea, and behind the cornea is a small space filled with aqueous humor, which helps maintain proper pressure inside the eye. Behind this liquid-filled space is the iris (the colored part of the eye), which surrounds the pupil. Behind the pupil is the lens, and behind the lens is a space filled with a "jellylike" material called vitreous humor. At the very back of the eye is the retina, with an area called the macula at its center. The optic nerve connects the back of the eye to the visual center in the brain.

Light is focused through the cornea and lens onto the retina (the cornea performs about 70% of the work in this process). Muscles in the pupil regulate how much light reaches the lens, and the lens can change shape depending on how near or far away the object of focus lies. The retina is the light-sensitive part of the eye, and cells there called photoreceptors transform light into energy that's transmitted to the brain via the optic nerve.

Uncorrected refractive errors

Problems with the shape of the eye's lens or cornea, known as refractive errors, are common and can cause a variety of vision issues (via Kellogg Eye Center). Although refractive errors are correctable with glasses or contacts, unaddressed refractive errors are the leading cause of visual impairment and blindness globally (via the World Health Organization). 

As the Kellogg Eye Center explains, there are four types of refractive error. In myopia (nearsightedness), close-up objects are in focus but distance vision is blurry. Myopia is usually inherited and progresses as an individual ages. Hyperopia (farsightedness) is the reverse: Distance vision is sharp but close-up vision is blurry, although in advanced hyperopia vision may be blurry all the time. Hyperopia is also inherited, but the condition may actually improve during adulthood. In astigmatism, the front surface of the cornea is asymmetric and curves more in one direction. This causes vision that's blurry at all distances and which may appear wavy or distorted, like looking into a funhouse mirror. While people are born with myopia, hyperopia, and astigmatism, presbyopia occurs later in life. In our forties, the lenses in our eyes begin to harden and can't change their shape as easily to focus on closeup objects. As a result, reading may become more difficult, requiring reading glasses.

Cataracts

As the Cleveland Clinic explains, cataracts occur when the clear lenses of the eye become cloudy. The lens is made mostly of water and proteins, and as they age these proteins break down, leaving fragments that cloud the lens. Cataracts usually develop slowly, but certain risk factors such as diabetes, use of steroid medications, or too much sun exposure may cause them to appear earlier. Cataracts cause blurry, hazy, or foggy vision, as well as sensitivity to bright light. They may also impair night vision and cause glare (a halo effect around an object, particularly at night). A change in eyeglass prescription may be enough to combat mild symptoms of early cataracts, but once cataracts become advanced and really begin to affect a person's ability to function normally, surgery is performed. In cataract surgery, the clouded lens is replaced with a clear artificial one. Although it sounds scary, cataract surgery is one of the most commonly performed surgeries. It's extremely safe and has a quick recovery time.

Cataracts are very common. According to the American Academy of Ophthalmology, more than 24.4 million Americans over 40 have them. Approximately 50% of Americans have developed cataracts by the time they reach age 75. Globally, 94 million people are blind or visually impaired because they have uncorrected cataracts (via the World Health Organization).

Glaucoma

According to the American Academy of Ophthalmology, glaucoma is a condition in which pressure inside the eye increases, injuring the optic nerve. Aqueous humor, the liquid at the front of the eye, maintains intraocular pressure (IOP). As new aqueous humor enters the eye, an equal amount leaves the eye through an area called the drainage angle. There are two types of glaucoma: open-angle and closed-angle (narrow-angle). In open-angle glaucoma (OAG), the fluid doesn't drain as rapidly enough. In closed-angle glaucoma (CAG), the iris causes an obstruction to the drainage angle. This blockage may happen gradually over time or very quickly (known as an acute attack). The latter is considered a medical emergency. Whatever the type of glaucoma, the rise in IOP puts pressure on the optic nerve, damaging the nerve fibers, causing vision loss and, eventually, blindness. Early OAG has no symptoms, while those with more advanced OAG will notice blind spots in their peripheral vision. If CAG happens gradually, individuals are unlikely to experience symptoms until they have an acute attack. During an acute attack of CAG, people can experience blurry vision as well as seeing halos or rainbows. They may also have severe eye pain, headaches, nausea, and vomiting.

While the optic nerve damage caused by glaucoma is permanent, medicated eye drops, laser treatments, and surgery can prevent further damage. Even so, among people 60 and over, glaucoma is one of the leading causes of blindness.

Age-related macular degeneration

Age-related macular degeneration (AMD) is a condition in which central vision becomes blurry because of damage to the macula — a part of the retina that controls sharp, straight-ahead vision (via National Eye Institute). AMD doesn't lead to total blindness, however. AMD comes in two forms: the more common dry form and the more severe wet form. In dry AMD, the macula thins gradually over time. In wet AMD, abnormal blood vessels develop in the retina, damaging the macula. Early dry AMD has no symptoms, and many people with intermediate dry AMD also don't notice any vision changes. Others may experience mild blurriness or difficulty seeing at night. Those with late dry AMD or wet AMD can experience blurriness in the center of their vision, blank spots, or colors may seem less bright. While there's no treatment for dry AMD, specially formulated supplements may help slow down the progression. For those with wet AMD, further vision loss can be prevented with medications injected directly into the eye and/or laser therapy.

AMD is very common. According to the American Academy of Ophthalmology, more than 9 million Americans have early AMD, while approximately 2 million people over age 50 have late AMD. Roughly 10% of Americans 80 and over have late AMD. Late AMD is more common in women than men.

Eye floaters

According to the Mayo Clinic, eye floaters appear as dark specks or strings in your field of vision. They move when you move your eyes, so they may seem to shift away when you're trying to look at them. These eye floaters are most often the result of age-related changes to the eye. As we age, the vitreous humor (the jellylike substance between the lens and the retina) loses thickness and becomes more liquid-y, shrinks, and pulls away from the interior surface of the eyeball. During this process, microscopic fibers in the vitreous can clump together and interfere with the passage of light through the eye. In some cases, floaters may be caused by inflammation or bleeding in the eye, or may be the result of eye surgery or certain medications injected directly into the eye.

In most cases, age-related floaters are benign and don't need to be addressed, although if they're caused by an underlying issue such as bleeding or inflammation, it's important to treat that issue. If floaters are seriously impacting an individual's vision, an ophthalmologist can use lasers to break up the floaters or can perform surgery to remove the vitreous and replace it with a synthetic solution. If you experience a sudden onset of new floaters or notice many more floaters than usual, this may be a sign that your retina has detached, which is a medical emergency.

Detached retina

While the shrinking of the vitreous that causes age-related floaters isn't usually a problem, in some cases the vitreous can stick to the retina and tear it (per American Academy of Ophthalmology). If the retina is torn, fluid can then get through and move behind the retina, causing it to lift away from the back of the eye. Symptoms of a detaching retina include the sudden appearance of many new eye floaters, seeing flashes of light, darkness in your peripheral vision, or a gray curtain covering part of your field of vision. While a detached retina can happen to anyone, those who are nearsighted, have experienced serious eye injury in the past, take glaucoma medication, or have had certain eye surgeries are at increased risk. Detached retinas also tend to run in families, so if you have close relatives who've had a detached retina, you're more likely to have one yourself. A detached retina needs to be addressed quickly, as it can lead to complete vision loss in that eye. An ophthalmologist can perform several types of surgery to treat a detached retina. These surgeries use different methods to press the retina back into place, where it can heal and reattach.

Approximately 28,000 retinal detachments occur in the United States each year, and about 1 in 300 individuals will experience a detachment during their lifetime (via Medscape).

Diabetic eye disease

Diabetes is a major public health issue in the United States. According to the American Diabetes Association (ADA), 37.3 million Americans (more than 11% of the population) have diabetes. Poorly managed diabetes can lead to a number of serious health complications, including vision loss.

As the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) explains, over time, uncontrolled high blood sugar injures the blood vessels at the back of the eye. This can lead to a cluster of conditions collectively known as diabetic eye disease. These conditions include glaucoma, cataracts, diabetic retinopathy, and diabetic macular edema. While cataracts and glaucoma are relatively common eye diseases that can affect anyone, they're twice as common in those with diabetes. According to the NIDDK, "in early diabetic retinopathy, blood vessels can weaken, bulge, or leak into the retina ... If the disease gets worse, some blood vessels close off, which causes new blood vessels to grow, or proliferate, on the surface of the retina ... These abnormal new blood vessels can lead to serious vision problems." About a third of diabetics over age 40 has some degree of diabetic retinopathy. Diabetes can also cause swelling (edema) of the macula, causing vision impairment when performing tasks such as reading and driving. The treatment for these conditions varies and can include medication, surgery, or laser treatments. What's most important, however, is proper management of blood sugar levels to prevent further damage.

Optic neuritis

According to the Mayo Clinic, optic neuritis is a condition in which inflammation damages the optic nerves that transmit information from the eye to the brain. Experts believe optic neuritis occurs when the immune system goes haywire and attacks the protective myelin sheath surrounding the optic nerve. A number of autoimmune conditions can trigger optic neuritis. Most significantly, optic neuritis is often the first symptom of multiple sclerosis (MS). Those who've had one episode of optic neuritis have about a 50% chance of developing MS during their lifetime. In addition to MS and several other rarer autoimmune conditions, optic neuritis may be triggered by bacterial infections (including Lyme disease, cat scratch fever, and syphilis) and viral infections (including measles, mumps, and herpes).

Symptoms of optic neuritis include loss of central and/or peripheral vision, decreases in visual acuity, reduced ability to distinguish color, and seeing flashing lights. It's also very common to have eye pain, especially when moving the eyes. Vision loss usually appears over hours or days. In most cases, while some damage to the optic nerve is permanent, changes in vision are usually temporary and normal vision returns within weeks or months. In some cases, however, vision loss may be permanent. Optic neuritis usually resolves on its own without treatment, although steroid medications may be given to help speed up the process.

Vitamin A deficiency

Vitamin A is known as the micronutrient that protects your peepers, so it's not surprising that severe deficiency can lead to vision loss. As the Linus Pauling Institute notes, vitamin A is required for the normal eye development in all mammals. A form of vitamin A called all-trans-retinol accumulates in the retina. Thanks to a complex chain of chemical reactions, it's this vitamin A in the retina that allows visual signals to reach the brain via the optic nerve. In addition to making vision in low light possible, vitamin A is also needed for the proper functioning of the eyes' cone cells, which perceive color.

Vitamin A deficiency can lead to night blindness, which begins as difficulty seeing clearly in low light and can progress to total blindness at night. Xerophthalmia is another ocular manifestation of vitamin A deficiency. In this condition, the eyes become very dry and crusted, which damages the cornea and retina. Similarly, keratomalacia is another complication of vitamin A deficiency and involves the drying and clouding of the cornea. Bitot spots — an accumulation of keratin in the eyes — can also occur, causing hazy vision (via WebMD). Although vitamin A deficiency are rare in the United States and other developed countries, it's a major public health concern worldwide, particularly among children. According to the WHO, an estimated 250,000–500,000 vitamin A-deficient children become blind every year. In fact, vitamin A deficiency is the leading cause of preventable childhood blindness globally.

Trachoma

According to the WHO, trachoma is an eye condition caused by repeated infections with the bacteria Chlamydia trachomatis. Although trachoma isn't a concern in the United States, it causes approximately 1.4% of all cases of blindness globally and is the leading infectious cause of vision loss. The bacteria is widespread in very poor, rural areas throughout Africa, Central and South America, Asia, Australia, and the Middle East. In these areas, children usually get their first infection around preschool age. In fact, 60–90% of young children in these areas have an active infection. The infection is easily spread by direct or indirect contact with discharge from an infected person's eyes or nose. Sharing clothing or bedding, as well as certain species of flies, can spread the infection.

Although the immune system can easily clear a single infection, individuals living in areas where trachoma is endemic are likely to be re-infected many times over their lifetime. These repeated infections can cause the inside of the eyelids to become so scarred that they turn inward, causing the eyelashes to rub against the eyeball. In addition to causing pain and light sensitivity, this rubbing can lead to severe scarring of the cornea. Vision loss or blindness from trachoma usually occurs between ages 30 and 40. Although antibiotics can clear the infection and surgery can be useful in advanced stages of the disease, blindness from trachoma is irreversible, and the hardest hit areas don't have easy access to these treatments.

Eye cancer

Although it's not as common as many other types, cancer can start in or spread to the eyes, causing vision loss. According to Cancer.net, "eye cancer is a general term used to describe many types of tumors that can start in various parts of the eye. It occurs when healthy cells in or around the eye change and grow uncontrollably, forming a mass called a tumor." Tumors can be benign or malignant (cancerous), but both can cause vision loss. Cancer that forms in the eyeball is referred to as intraocular cancer. Eye cancer can either start in the eye (primary) or spread to the eye from some other part of the body (secondary). Intraocular melanoma is the most frequently seen form of primary eye cancer. Other types of primary eye cancer include intraocular lymphoma, retinoblastoma, hemangioma, and eyelid carcinoma.

Symptoms of eye cancer can vary significantly from person to person but often include vision changes such as reductions in field of vision or seeing flashes of light, spots, squiggly lines, or floaters (via Cancer.net). In some cases — such as if the cancer is very slow-growing and the person has no symptoms — doctors may opt for an "active surveillance" approach to monitor the cancer. In other cases, radiation therapy or laser treatment may be helpful. Surgery to remove part or all of the affected eye is another treatment option. These treatments may also cause temporary or permanent vision loss (via Cancer.net).

Stroke

Although most conditions that cause vision loss affect the eye itself, conditions that impact the visual processing centers of the brain can also lead to visual impairment. According to the Wilmer Eye Institute at Johns Hopkins, three parts of our brain are involved in making sense of the information our eyes take in. The occipital lobes are generally considered the centers of visual processing, although the temporal lobes and parietal lobes are essential for our visual-spatial awareness. Each side of our brain has these three lobes, and the lobes on the right process information from the left eye, while the lobes on the left process information from the right eye. If a stroke causes damage to any of these areas, it can lead to temporary or permanent changes in vision. Because every stroke is unique, the nature and extent of vision problems after a stroke varies widely.

The Mayo Clinic identifies three types of stroke. Ischemic strokes are the most common and occur when the blood vessels supplying the brain become blocked with arterial plaque or a clot. A transient ischemic attack (TIA), also known as a "mini-stroke," is similar, but the blockage is only temporary and does not cause permanent brain damage. A hemorrhagic stroke occurs when a blood vessel leaks or ruptures. According to the CDC, roughly 7.8 million Americans (3.1% of the population) have had at least one stroke in their lifetime.

Eye injuries

While most conditions that can lead to vision loss happen gradually over time, vision loss can also happen in the blink of an eye (pun intended). Like any other part of the body, the eyes are susceptible to acute injury and trauma. According to the American Academy of Ophthalmology, an estimated 2.4 million eye injuries occur in the United States each year, with approximately 35% of these injuries occurring in those between the ages of 18 and 45. Of the 2.4 million eye injuries, roughly 35% involve a foreign object in the eye, while another 25% are open wounds, 25% are contusions (bruises), and the remaining 15% are burns. About 125,000 eye injuries involve common household products, while more than 2,000 Americans sustain an eye injury at work each day. About 90% of eye injuries could be prevented by wearing proper eye protection.

Because there are such a wide variety of ways to injure the eye, an injury's impact on vision can be highly variable. People who've experienced eye trauma may see floating black spots, flashes of light, blurry vision, or double vision (via the Cleveland Clinic). These changes may be temporary or permanent. Depending on the extent of damage, vision may be lost entirely. Pain, bruising and swelling, bleeding inside the eyeball, and difficulty moving the eyes can also occur. Wearing proper eye protection when engaging in activities that might lead to eye injury is critical.